Question Which double tank size + which type of wing?

[DutchDiverDude]

Hi all,

With around 60 dives in the cold Dutch weather, I'm leaning towards going to a 'redundant' setup, so 2 1st and 2nd stages.
My first thought was to replace the valves of my single 12L bottles with double valves, but that still means that I have a single tank without reduncency and a max volume of 12L. As most advanced divers here (at least that I know) dive doubles, that's what I'm looking into. However, in the small group of fellow divers I'm in, I hear lot's of conflicting opinions. I was wondering if anyone here could clear some things up. Some like double 8.5L, and hate the double 12L, others like double 12L. Then there's also double 7L's and double 10L. What is everyone's opinion on the size of the tanks? Please keep it in liters, I don't understand the other way of measuring haha.
Another thing that confuses me is the wing. I watched some video's on wings and understand the different types, but I cannot figure out what 'lift weight' wing I would need with which type of double tank.

My setup right now:
Compressed neoprene drysuit with undersuit
BCD with 12L single tank.
15Kg of lead, divided over pockets and belt.

My dives right now are regular cold water divers still within AOW limits, but I plan on continuing training here to later also do more specialties, probably tech diving as well in the future. I surface with 90/100 bar in the tank, which is filled to about 200/210 bar.
Here's the opinions of my buddies on the tank sizes.
Double 7: garbage, might as well do single 15L in that case. Only advantage is redundant tanks.
Double 8.5: quite popular here. Most guys that use these actually have more than one set of tanks, and use these for the 'recreational' dives and training dives with their students. They like that they can get 2 dives out of the set without too much weight.
Double 10: I don't know anyone who has this.
Double 12L: Apparently good for everything, with the disadvantage of heavy weight. My instructor gets 3 normal dives out of a set of double 12's. Apart from that the benefit only becomes apparent at deeper / more technical dives.

So based on these opinions I'm considering double 8.5 and double 12, mainly because of weight versus benefits in the future.
However, I also carry 15Kg of lead with me due to a compressed neoprene drysuit. How would the total weight of my equipment change with either the double 8.5 or double 12? I'm mostly wondering if the added weight of the tanks means I need to carry less lead, and the difference of TOTAL weight between single 12L, double 8.5L and double 12L may actually not be that much....

And then the wing.... how much lbs of lift is needed considering either a 8.5 or 12L double set with neoprene drysuit?
And how does the backplate factor in there?
Are there disadvantages of having too much lift? Honestly this wing lift thing is the most confusing thing about the whole situation to me.

Thanks!

 

[Badger7]

Assuming you're currently diving a relatively balanced rig that (with your current exposure protection) puts you at around -4kg buoyancy with your single cylinder full and empty wing, and neutral buoyancy at 5m with an empty tank and empty wing, then you can just calculate the actual differences for a second tank (again, assuming you're diving the same backplate and harness and accessories now as you would be in a twinset configuration).

You'll first have to check the empty/full buoyancy characteristics of your specific cylinders. The 12l steel equivalents we use over here are very close to neutral buoyancy when empty, which means the second tank itself doesn't allow you to remove hardly any lead from the weight belt. You should be able to subtract the total weight of the steel tank bands/hardware from your lead requirements, the weight of the (second) 1st stage, and the weight of the isolation crossbar, as some back-of-the napkin math on how much lead you might be able to drop. Careful though - if you're using (and would be losing) an STA, particularly if it is weighted, that offsets some of the weight of the tank bands, so consider that as well.

On the other side of the equation, you'll need to ensure that wing you swap in for a doubles configuration has at least as much extra lift potential over and beyond your current singles wing as the weight of the gas in the second cylinder. You can trade extra steel for less lead, but carrying that extra air in the second tank will require more lift to keep you afloat at the start of the dive, assuming no other changes. Make sure it is a proper doubles wing, wide enough to be fully inflated when pinned between your backplate and two big 12l tanks.

 

[inquis]

You pretty much outlined it. The burden of carrying things around vs more time in-water (possibly split over multiple dives). The D8.5s will reduce risk of going OOG due to gear failure and fairly light but little other advantages. The D12s will reduce risk and open the door to going beyond NDL.

Either way, your lead requirements will drop. All else being equal, I drop 5 kg of ballast between a single 13L and D13s (or LP85s as they're called here in the US). The backplate also acts as ballast, so if you are using something else now, you could likely drop another 2-3 kg lead.

Total equipment weight carried will increase. As a data point, my D13s weigh 34 kg. My 13L+5 kg lead weigh 20 kg.

My suggestion would be the D12s, as that covers you now and the possible tech path. Plus multiple NDL dives without refills or buying multiple sets of 8.5s.

Lastly, a 20 kg wing will be fine for anything you're considering.

 

[Alekseolsen]

I've had both a D8.5, D12 and D18.

D8.5 works well if you do a single rec dive at a time. If you do two rec dives or anything with a ceiling, you'll like the D12's.

What I did like a lot with the 8,5's were how much easier it was getting used to shutdowns with them due to the reduced diameter of the bottles puts them close to your back.

Weight wise I wouldn't worry. All of them will be heavy when geared up. My D18 or the D8.5 were the same - heavy

 

[DutchDiverDude]

Thanks!
I'm considering the 2x12L more and more now.
The only thing I'm not sure about right now, is to either stick with my current BCD (not a wing) for now and replace the single valve with a double valve (using double valves is mandatory in some places I visit) on my 2 12L tanks so that I am set for most of my dives the coming year or so. I wait until I can find a nice 2nd-hand set, and worst case I cannot do some of the deeper dive trips my club does (North sea diving for example, where they really recommend higher volumes of air than a single 12L tank).
Or I skip the double valve on my single 12L's and get a double 12L soon....

I see with most guys at our club that they either dive single 12L, or doubles. They don't really switch it up, even though they still have single 12L tanks at home. In that way putting the double valves on the 12L tanks is kind of like throwing away money if I already know that I want to go to double 12L's...

 

[andromedakun]

Hi Dutchie,

I was recently in the same situation as you about going for doubles. I ended up going for double 12L's as I managed to grab a pair of Carbon Fiber tanks for cheap on a 2nd hand site.

Now, one important aspect that came up when discussing with other divers and trainers is how tall you are. I'm about 1,90 Meters tall and the consensus was that I needed at least 2x10L as smaller was going to be difficult to get to the manifolds and whatnot.

As I know Dutchies are fairly tall, this might also be an issue for you.

About the double output on the single tank, it might not be a waste if you use it sometimes. You can keep your regulators as you are used with your doubles (probably 1 regulator on each 1st stage) instead of having to change your regulator configuration every time you use that tank.

 

[Alex Ouroboros]

Double 232bar 12ltr (twin 12,s in the UK) are really no bother my wife loves hers, they work out well with a drysuit as only need 1-3kg of diving in the sea with this set up with a stainless backplate and usually 0-1kg in fresh,
A word of warning, many years ago there were tests done by some tec dive pros at the time, regarding the genuine redundancy properties of twin cylinders,
Firstly nowadays you either go independent (non manifolded) or manifolded with isolation valves, I have seen non manifolded “death traps” advertised so stay away from those,
Anyway back to the tests from a few years ago, the tests were done according to the rule of thirds, and working on the idea that the shutdown would be done after using a third of the gas with the aim of saving a third to get home on, with the redundant third gone during shutdown, it simulated a variety of situations (burst o ring being worst) the cut off point of being useful is twin 7ltr 232bar, when you need to do a shutdown on these cylinders on a dive you simply don’t have enough gas to get the drill done and save enough gas, twin 7ltr 300bar however do have enough gas, I realise your not going tec at the moment but I honestly think it’s false economy to not get twins and be able to shut down or get shutdowns done as this is the point of twins, the 12,s at 232 or 7,s at 300 from a weight - with little lead in a drysuit is the sweet spot in my opinion, however your personal build and fitness or mobility level may dictate otherwise, ideally borrow some and try first.
Best of luck I hope that’s helped a little.

Ps, the only weak link in working out gas volumes when considering the 300bar twins is being able to always get a 300bar fill… some in the uk can only give 240bar.
Also, my weapon of choice regarding backplate and wing is second hand stainless halcyon with Cinch and go for a donut wing, much easier to trim out, these can be had for about €500-550, and are fantastic and I’ve used many different types

 

[pisauron]

Double 232bar 12ltr (twin 12,s in the UK) are really no bother my wife loves hers, they work out well with a drysuit as only need 1-3kg of diving in the sea with this set up with a stainless backplate and usually 0-1kg in fresh,
A word of warning, many years ago there were tests done by some tec dive pros at the time, regarding the genuine redundancy properties of twin cylinders,
Firstly nowadays you either go independent (non manifolded) or manifolded with isolation valves, I have seen non manifolded “death traps” advertised so stay away from those,
Anyway back to the tests from a few years ago, the tests were done according to the rule of thirds, and working on the idea that the shutdown would be done after using a third of the gas with the aim of saving a third to get home on, with the redundant third gone during shutdown, it simulated a variety of situations (burst o ring being worst) the cut off point of being useful is twin 7ltr 232bar, when you need to do a shutdown on these cylinders on a dive you simply don’t have enough gas to get the drill done and save enough gas, twin 7ltr 300bar however do have enough gas, I realise your not going tec at the moment but I honestly think it’s false economy to not get twins and be able to shut down or get shutdowns done as this is the point of twins, the 12,s at 232 or 7,s at 300 from a weight - with little lead in a drysuit is the sweet spot in my opinion, however your personal build and fitness or mobility level may dictate otherwise, ideally borrow some and try first.
Best of luck I hope that’s helped a little.

Ps, the only weak link in working out gas volumes when considering the 300bar twins is being able to always get a 300bar fill… some in the uk can only give 240bar.
Also, my weapon of choice regarding backplate and wing is second hand stainless halcyon with Cinch and go for a donut wing, much easier to trim out, these can be had for about €500-550, and are fantastic and I’ve used many different types

I use twin 7L with 300bar.
Most fills are to 260 actually. Then you need to cool it off and then fill again and again maybe.


Anyway, I'm interested to hear about the test because i didn't get it. The shutdown drill takes a few seconds. During that time i lost third of my gas?
Ok. So I'm left with 1,200 liters?
(14*260)/0.33
Or if i had 300bar i have 1,400 liters left.

Shouldn't that be enough to rise to the surface?
But with 200 bars i only have 900 liters now. That's not enough?

 

[Alex Ouroboros]

I use twin 7L with 300bar.
Most fills are to 260 actually. Then you need to cool it off and then fill again and again maybe.


Anyway, I'm interested to hear about the test because i didn't get it. The shutdown drill takes a few seconds. During that time i lost third of my gas?
Ok. So I'm left with 1,200 liters?
(14*260)/0.33
Or if i had 300bar i have 1,400 liters left.

Shouldn't that be enough to rise to the surface?
But with 200 bars i only have 900 liters now. That's not enough?

Well it really depends on if your in deco or not, from memory the tests were done to settle an ongoing argument in the tec world from the 90,s I will trawl about and try and find some written evidence of it, the other interesting point i remember was the discussion of what looses gas quicker the disintegration of an o ring in the low? or high? pressure port of a first stage and I hadn’t realised until then that the opening in the HP port on a first stage is minuscule as it is literally just to get a pressure reading from so the gas loss of the HP is not anything like you would expect it to be, (if memory serves I believe it was less than the LP)

 

[Alex Ouroboros]

I use twin 7L with 300bar.
Most fills are to 260 actually. Then you need to cool it off and then fill again and again maybe.


Anyway, I'm interested to hear about the test because i didn't get it. The shutdown drill takes a few seconds. During that time i lost third of my gas?
Ok. So I'm left with 1,200 liters?
(14*260)/0.33
Or if i had 300bar i have 1,400 liters left.

Shouldn't that be enough to rise to the surface?
But with 200 bars i only have 900 liters now. That's not enough?

Ahhh, an extra point, your shutdown drill is in practise and of course it only takes a few seconds, these tests where done (again if memory serves) simulating an actual gas loss situation so I remember vividly that they tried with damaged, broken or just plain missing o rings in either the first stage din or A clamp fitting and then in the manifold system itself, it was actually done underwater and must have taken hours if not days, was very comprehensive, I also remember there was very little difference gas loss wise between a given o ring being damaged or plain not there…